1. Field of the Invention
The present invention relates to a carrier for electrophotographic developer, a developer using the carrier, a container for the developer, an image forming apparatus using the developer, an image forming method using the same and a preparation method of the carrier.
2. Description of the Related Art
There are proposed two developing methods in electrophotography; a so-called one-component developing method using one-component developer comprising merely toner as a main ingredient, and a two-component developing method using two-component developer comprising a mixture of a carrier made from materials such as glass beads, magnetic carrier, or those coated by resinous or other coatings, and a toner.
Two-component developing method is advantageous in comparison with one-component developing method, because it uses carrier which has large surface area causing good enough triboelectric-charge for toner, thereby charge of the toner is made stable, holding high quality in images for a long period of developing time. And as the two-component developing method shows a high ability in the supply of the toner to the developing area, there are many incidences having been employed particularly in high-speed apparatuses.
In digital electrophotographic system comprising steps of forming latent electrostatic images onto a photosensitive member by laser beam-irradiation and the like then developing the latent images, two-component developing method usable above characteristics is also being widely used.
Recently, size reduction and condensed distribution of dot units for latent image (pixcel units) have been designed to satisfy the requirements for improving the resolution degree, reproducibility of highlight image and faithful color-imaging. In, particular, important concern in the field is the achievement of developing system, which enables a faithful development of those latent images (dots composing each image). Therefore, many proposals were made from both points of processing means and developer (toner and carrier). As the processing means, a restriction of development gap and a slenderization of the layers composing photosensitive member are effective, however there are still remaining problems with regard to the processing means that the processing cost is increased as a result of such improvements, and sufficient reliability is not yet achieved, and the like.
On the other hand, with regard to developer, the dot reproducibility is considerably improved by use of small size of toner. However, problems are occurred with developer including small size of toners, such as a stain (or, in other word smear) in background area is generated, optical density in image is declined and others. And, in case of toner having small size which is used for full-color image, resins having a low softening-temperature are generally used which, in comparison with black toner, increase a spent amount on the surface of carrier, and degrade the quality of developer by time lapse and show a tendency apt to toner-scattering and to stain background area.
Various proposals for use of small size carriers are also proposed. For example, Japanese Patent No. 2832013 discloses a developing method for reversal development of a latent electrostatic image formed on a latent image-bearing member having organic photo-conductive layer, using magnetic brash of two-component developer which is held on a developer-bearing member and contains a toner capable of being charged in the same polarity as that of the latent image and a carrier; with imposing biased electric field having alter current component and direct current component, wherein the carrier is a carrier including ferrite core-material particles, the core-material particles are being coated with a electric insulating-resin and having a weight-average diameter ranging from 30 to 65 xcexcm after coated with the resin, and the core-material particles have, at their surfaces, small bores of 1500 xc3x85 to 30000 xc3x85 in average size.
And Japanese Patent No. 3029180 discloses a carrier for electrophotographic developer using carrier particles, wherein the carrier particles have a size ranging from 15 to 45 xcexcm in 50%-average diameter(D50) (the D50 presents a particles amount summed up every ingredient divisions by size till becomes to 50%), the content of smaller carrier particles less than 22 xcexcm in size ranges from 1 to 20%, the content of small carrier particles less than 16 xcexcm in size is not higher than 3%, the content of large carrier particles more than or equal to 62 xcexcm in size ranges from 2 to 15%, the content of larger carrier particles more than 88 xcexcm in size is not higher than 2%, and the carrier satisfies a ratio(S1/S2) of surface area(S1) measured by air-permeation method in comparison with surface area(S2), a range represented by;
1.2xe2x89xa6(S1/S2)xe2x89xa62.0
where the S2 represents surface area(S2) calculated from following Equation 1;
S2=(6/xcfx81xc2x7D50)xc3x97104xe2x80x83xe2x80x83Equation 1
(where, the xcfx81 is specific gravity of the carrier).
Further, Japanese Unexamined Patent Publication of Tokkai Hei 10-198077 discloses a carrier for developer used for developing electrostatic latent image, wherein a 50%-average diameter(D50) in volumetric average diameter of the carrier ranges from 30 to 80 xcexcm , a ratio(D50/D10) of the D50 for a 10%-average diameter(D10) in volumetric average diameter of the carrier is 1.8 or less, a ratio (D90/D50) of a 90%-average diameter(D90) in volumetric average diameter of the carrier for the 50%-average diameter (D50) in volumetric average diameter of the carrier is 1.8 or less, the magnetic moment (at 1 kilo Oe of magnetic field) of the carrier ranges from 52 to 65 emu/g.
By the use of this kind of carriers having small diameter gives following benefits;
(1) Surface area per unit volume is large, therefore they can give good enough triboelectric-charge for each toner, and scarcely yield toners which have a low level of electric-charge and reverse polarity-charge too, accordingly, scattering of toner particles at the periphery of dot for image-forming and smear (blurring) in background area are few, thus dot reproducibility is excellent;
(2) Due to the nature of large surface area per unit volume and scarce to generate the smear in background area, a low level of average electric-charge in toner is allowable to use, notwithstanding, a high optical density of image is obtained, thus carrier of small diameter is capable to compensate the shortcomings which are caused by use of stall size of toner, hence is effective for driving out the advantages of small size of toner;
(3) As small diameter of carrier forms a dense magnetic brush and the head of the formed magnetic brush has an excellent fluidity, accordingly the trace drawn by dragging of the head of the magnetic brush on image is hardly imprinted.
However, carriers of small diameter in prior arts have a important problem that they are apt to deposit themselves on surfaces contacted with the developer, thus brings flaws on photosensitive member or fixing roller, therefore was difficult to utilize in practical.
With regard to carriers of small diameter, we, the inventor, have investigated diameters of carriers deposited on the surfaces of photosensitive member, and found out the facts that, out of used original carrier having a size-distribution, the smaller size of carrier particles show a tendency apt to deposit on the surface of photosensitive member preferentially, and the deposited ratio of smaller size of particles less than 22 xcexcm diameter was overwhelmingly much in all deposited particles.
Accordingly, the first object of the present invention is to provide a carrier for electrophotographic developer and a developer using the same which are able to produce high quality of image-reproductions having an excellent dot-reproducibility, an excellent highlight-reproducibility, a high optical density of image, and showing a scarce or devoid of smear in background area.
The more object of the present invention is to provide a container for the developer.
The further object of the present invention is to provide an image-forming apparatus that is loaded the container for the developer.
The furthermore object of the present invention is to provide a preparation method of the carrier.
Above and other objects are achieved by the present invention comprising;
(1) A carrier for electrophotographic developer comprising carrier particles, each carrier particle comprising a magnetic core particle and a resin layer formed on the surface of said magnetic core particle, and said carrier having a magnetic moment of 76 emu/g or more at 1 KOe, and said carrier particles having a weight-average particle diameter (Dw) in a range of 25 to 45 micrometer, and said carrier particles comprising:
(1) carrier component particles having a diameter of less than 44 micrometer in an amount of 75 wt. % or more,
(2) carrier component particles having a diameter of 62 micrometer or more in an amount of less than 1 wt. %, and
(3) carrier component particles having a diameter of less than 22 micrometer in an amount of 7.0 wt. % or less, based on the total amount of said carrier particles;
(2) A carrier for electrophotographic developer according to above paragraph (1), wherein said carrier component particles hang a diameter of less than 22 micrometer is in an amount of 3.0 wt. % or less;
(3) A carrier for electrophotographic developer according to above paragraph (1), wherein said carrier component particles having diameter of less than 22 micrometer is in an amount of 1.0 wt. % or less;
(4) A carrier for electrophotographic developer according to above paragraph (1), wherein the bulk density of the carrier is in degree of 2.2 g/cm3 or more;
(5) A carrier for electrophotographic developer according to above paragraph (1), wherein the specific electric-resistance denoted by (log Rxc2x7cm) of the carrier is in a value of 12.0 or more;
(6) A carrier for electrophotographic developer according to above paragraph (1), wherein the magnetic core particle is a MnMgSr ferrite material;
(7) A carrier for electrophotographic developer according to above paragraph (1), wherein the magnetic core particle is a Mn ferrite material;
(8) A carrier for electrophotographic developer according to above paragraph (1), wherein the magnetic core particle is a magnetite material;
(9) A carrier for electrophotographic developer according to above paragraph (1), wherein the resinous coating layer is a silicone resin coating layer;
(10) A carrier for electrophotographic developer according to above paragraph (1), wherein the resin layer comprises a resin layer containing a reaction product by amino silane type of coupling agents.
Further, above and other objects are also achieved by the present invention including;
(11) An electrophotographic developer using a toner and a carrier comprising carrier particles, each carrier particle comprising a magnetic core particle and a resin layer formed on the surface of said magnetic core particle, and said carrier having a magnetic moment of 76 emu/g or more at 1 KOe, and said carrier particles having a weight-average particle diameter (Dw) in a range of 25 to 45 micrometer, and said carrier particles comprising:
(1) carrier component particles having a diameter of leas than 44 micrometer in an amount of 75 wt. % or more,
(2) carrier component particles having a diameter of 62 micrometer or more in an amount of less than 1 wt. %, and
(3) carrier component particles having a diameter of less than 22 micrometer in an amount of 7.0 wt. % or less, based on the total amount of said carrier particles;
(12) An electrophotographic developer using a toner and a carrier according to above paragraph (11), wherein the toner charge per mass is less than or equal to 35 xcexcc/g at coverage ratio 50% on the surface of the carrier by the toner;
(13) An electrophotographic developer using a toner and a carrier according to above paragraph (11), wherein the toner charge per mass at 50% is less than or equal to 25 xcexcc/g at coverage ratio 50% on the surface of the carrier by the toner;
(14) An electrophotographic developer using a toner and a carrier according to above paragraph (11), wherein the toner has a weight-average particle-diameter of less than or equal to 6.0 xcexcm.
Furthermore, above and other objects are also achieved by the present invention including;
(15) A container for electrophotographic developer using a toner and a apparatus loaded with a carrier comprising carrier particles, each carrier particle comprising a magnetic core particle and a resin layer formed on the surface of said magnetic core particle, and said carrier having a magnetic moment of 76 emu/g or more at 1 KOe, and said carrier particles having a weight-average particle diameter (Dw) in a range of 25 to 45 micrometer, and said carrier particles comprising;
(1) carrier component particles having a diameter of less than 44 micrometer in an amount of 75 wt. % or more,
(2) carrier component particles having a diameter of 62 micrometer or more in an amount of less than 1 wt. %, and
(3) carrier component particles having a diameter of less than 22 micrometer in an amount of 7.0 wt. % or less, based on the total amount of said carrier particles.
Still further, above and other objects are also achieved by the present invention including;
(16) An image forming apparatus loaded with a container for electrophotographic developer, wherein the developer uses a toner and a carrier comprising carrier particles, each carrier particle comprising a magnetic core particle and a resin layer formed on the surface of said magnetic core particle, and said carrier having a magnetic moment of 76 emu/g or more at 1 KOe, and said carrier particles having a weight-average particle diameter (Dw) in a range of 25 to 45 micrometer, and said carrier particles comprising:
(1) carrier component particles having a diameter of less than 44 micrometer in an amount of 75 wt. % or more,
(2) carrier component particles having a diameter of 62 micrometer or more in an amount of less than 1 wt. %, and
(3) carrier component particles having a diameter of less than 22 micrometer in an amount of 7.0 wt. % or less, based on the total amount of said carrier particles.
Still further, above and other objects are also achieved by the present invention including;
(17) An image forming method using an electrophotographic developer, wherein the developer uses a toner and a carrier comprising carrier particles, each carrier particle comprising a magnetic core particle and a resin layer formed on the surface of said magnetic core particle, and said carrier having a magnetic moment of 76 emu/g or more at 1 KOe, and said carrier particles having a weight-average particle diameter (Dw) in a range of 25 to 45 micrometer, and said carrier particles comprising:
(1) carrier component particles having a diameter of less than 44 micrometer in an amount of 75 wt. % or more,
(2) carrier component particles having a diameter of 62 micrometer or more in an amount of less than 1 wt. %, and
(3) carrier component particles having a diameter of less than 22 micrometer in an amount of 7.0 wt. % or less, based on the total amount of said carrier particles.
Still further, above and other objects are also achieved by the present invention including;
(18) A preparation method of a carrier for electrophotographic developer comprising carrier particles, each carrier particle comprising a magnetic core particle and a resin layer formed on the surface of said magnetic core particle, and said carrier having a magnetic moment of 76 emu/g or more at 1 KOe, and said carrier particles having a weight-average particle diameter (Dw) in a range of 25 to 45 micrometer, and said carrier particles comprising:
(1) carrier component particles having a diameter of less than 44 micrometer in an amount of 75 wt. % or more,
(2) carrier component particles having a diameter of 62 micrometer or more in an amount of less than 1 wt. %, and
(3) carrier component particles having a diameter of less than 22 micrometer in an amount of 7.0 wt. % or less, based on the total amount of said carrier particles;
and comprising steps of (i) classifying a magnetic material of finely pulverized particles, thereby obtaining a core material of particles having a weight-average particle-diameter(Dw) of the carrier ranges from 25 to 45 xcexcm, the content of particles having diameter less than 44 xcexcm is more than or equal to 75% by weight, the content of particles having diameter more than or equal to 62 xcexcm is less than one percent by weight, the content of particles having diameter less than 22 xcexcm is less than or equal to 7.0% by weight, the magnetic moment of the carrier at 1 kilo Oe of magnetic field is more than or equal to 76 emu/g, (ii) providing a resinous film onto the magnetic core material;
(19) A preparation method of a carrier for electrophotographic developer comprising carrier particles, each carrier particle comprising a magnetic core particle and a resin layer formed on the surface of said magnetic core particle, and said carrier having a magnetic moment of 76 emu/g or more at 1 KOe, and said carrier particles having a weight-average particle diameter (Dw) in a range of 25 to 45 micrometer, and said carrier particles comprising:
(1) carrier component particles having a diameter of less than 44 micrometer in an amount of 75 wt. % or more,
(2) carrier component particles having a diameter of 62 micrometer or more in an amount of less than 1 wt. %, and
(3) carrier component particles having a diameter of less than 22 micrometer in an amount of 7.0 wt. % or less, based on the total amount of said carrier particles;
and comprising steps of (i) providing a resinous film onto a magnetic core material of finely pulverized particles, (ii) classifying the magnetic core material of finely pulverized particles having resinous film thereon, thereby obtaining a core material of particles having a weight-average particle-diameter(Dw) of the carrier ranges from 25 to 45 xcexcm, the content of particles having diameter less than 44 xcexcm is more than or equal to 75% by weight, the content of particles having diameter more than or equal to 62 xcexcm is less than one percent by weight, the content of particles having diameter less than 22 xcexcm is less than or equal to 7.0% by weight, the magnetic moment of the carrier at 1 kilo Oe is more than or equal to 76 emu/g;
(20) A preparation method of the carrier according to above paragraph (18), wherein a vibration sieve equipped with an ultrasonic wave-generator is used in the step of (i) classifying a magnetic material of finely pulverized particles;
(21) A preparation method of the carrier according to above paragraph (19), wherein a vibration sieve equipped with an ultrasonic wave-generator is used in the step of (ii) classifying the magnetic core material of finely pulverized particles having a resinous film thereon;
(22) A preparation method of the carrier according to above paragraph (20) or (21), wherein a vibration sieve, which is quipped with an ultrasonic wave-generator and a resonator ring to transfer ultrasonic waves generated by the ultrasonic wave-generator to the vibration sieve, is used in the step of classifying a magnetic material of finely pulverized particles.
The carrier for electrophotographic developer (it some times may merely described as the carrier hereinafter) of the present invention consists of a core material of magnetic particles and thereon provided a resinous layer.
With regard to the carrier of the present invention, weight-average particle-diameter(Dw) thereof ranges from 25 xcexcm to 45 xcexcm, favorably from 30 xcexcm to 45 xcexcm. Larger weight-average particle-diameter(Dw) than above range is hard to deposit the carrier, however the condensation of toner content in the developer for the sake of obtaining a high optical density of image increases smear (stain) of background area abruptly, and causes a large variance of dot diameter in case of development of small dot for latent image.
The carrier deposition in the present invention means a phenomenon of depositing carrier onto electrostatic latent image area or background area. The stronger electric field at both areas shows more abundance deposition of carrier. However image area is as a rule decreased in the strength of electric field by developing with toner, therefore is hard to deposit carrier in comparison with background area. As described forgoing, carrier deposition causes the flaws on photosensitive member or fixing roller, thus is unfavorable.
In the carrier of the present invention, the content of particles having diameter less than or equal to 44 xcexcm is more than or equal to 70% by weight, favorably more than or equal to 75% by weight. And the content of particles having diameter less than or equal to 44 xcexcm is favorably less than or equal to 95% by weight, more favorably less than or equal to 90% by weight. Within the content ratio of less than or equal to 95% by weight, one can obtained a carrier having desired size without excess expenditure.
The content of particles having diameter more than 62 xcexcm is less than three percent by weight, favorably less than one percent. And the content of particles having diameter more than 62 xcexcm is more than 0.3 percent by weight. Within the content ratio of more than or equal to 0.3% by weight, one can obtained a carrier having desired size without excess expenditure.
The larger content of particles having diameter more than 62 xcexcm causes more noticeable scattering in dot sizes of latent image. It is thought that this tendency depends upon the fact that, in case of carrier particles having diameter more than 62 xcexcm, small increase in size significantly influences to the reduction of percentage by weight of carriers having diameter ranging from 44 xcexcm to 62 xcexcm. The content of particles having diameter less than 22 xcexcm is less than seven percent by weight, favorably less than three percent, more favorably less than one percent. The content of particles having diameter less than 22 xcexcm is less than seven percent by weight, favorably less than three percent, more favorably less than one percent. And the content of particles having diameter less than 22 xcexcm is more than or equal to 0.1 percent by weight. Within the content ratio of more than or equal to 0.1% by weight, one can obtained a carrier having desired size without excess expenditure.
In case of carrier having small diameter(size), a majority of carriers deposited are those fine particles having diameter less than 22 xcexcm . We have evaluated carrier depositions with varied content ratios of smaller particles less than 22 xcexcm diameter in small carrier particles of size ranging from 25 xcexcm to 45 xcexcm, and arrived to a conclusion that no serious problem is presented in the case of content of small particles less than 22 xcexcm diameter is less than or equal to seven percent, and carrier deposition is improved by the content less than or equal to three percent, and more improved by restrict the content to a level less than or equal to one percent.
At the same time, it was found out that carrier deposition is also substantially evaded by controlling the magnetic moment of the carrier at 1 kilo Oe, to a level more than or equal to 76 emu/g.
The carrier of the present invention can prepared by pulverizing a magnetic material, classifying the finely pulverized particles so as to obtain a core material of particles having the defined average particle-diameter(Dw) and the defined distribution in average particle-diameters of the particles, then providing a resinous film onto the classified magnetic core material.
Above classification includes pneumatic classification, screen classification and the like. Vibration sieves have been used favorably, however conventional vibration sieve shows an inconvenience that it is apt to occur mesh-straggle (clogging) in case of classification for small size of particles, thus making inferior operation efficiency.
We have studied to develop a method capable of removal small size of particles with high efficient, and arrived to a result that small particles less than 22 xcexcm diameter are removed efficiently and sharply by adding a vibration using ultrasonic wave to screen mesh in screen classification process.
The ultrasonic wave-vibration for vibrating the screen mesh can be obtained by giving an electric power of high frequency to a converter (transducer) which uses PZT vibrator and converts electric power to a ultrasonic wave generating vibration power. For making a vibration in screen mash, vibration of ultrasonic wave is transferred to a resonator member which is being fixed to the screen mesh, and the resonator member is resonated with the vibration of ultrasonic wave to make vibration for the screen mesh. Frequency of the ultrasonic wave for vibrating the screen mesh ranges from 20 to 50 K Hz, 30 to 40 K Hz is favorable. Form of the resonator member is allowed to be any one suitable to make vibration in the screen mesh, and generally is a ring form. Direction to make vibration of the screen mesh is favorably perpendicular to the surface of the screen mesh.